Tire testing apparatuses used for special inspection, evaluation, development and the like are equipped with multi-component force gages capable of measuring forces (Fx, Fy, Fz) along orthogonal three axes applied to tires rotating in contact with a road surface or moments (Mx, My, Mz) around the axes. Such a multi-component force gage is always calibrated when it is incorporated into the tire testing apparatus for the first time. Further, even after the incorporation into the tire testing apparatus, the multi-component force gauge is continually calibrated, for example, at predetermined regular time intervals to maintain its accuracy in measurement of the forces or the moments.
For example, Patent Document 1 discloses a calibration method for calibrating a multi-component force gauge itself and a jig or a device for performing calibration. In the calibration method, a weight is attached to the multi-component force gauge through a rope and a pulley (a tackle) to thereby exert a force given by the weight or a moment generated by the weight on the multi-component force gauge, and the multi-component force gauge is calibrated accordingly. The calibration method is intended to separately calibrate the multi-component force gauge as a single unit removed from a tire testing apparatus, and not designed for calibrating the multi-component force gage that remains incorporated in the tire testing apparatus.
On the other hand, there has been a calibration apparatus for calibrating a multi-component force gauge that is in a state incorporated in a tire spindle. For example, FIG. 4 shows such a calibration apparatus for calibrating the multi-component force gauge that is in the state incorporated in the tire spindle in a tire testing apparatus in which the multi-component force gauge is incorporated in the tire spindle rotatably including a spindle shaft for retaining a tire. The calibration apparatus is employed at an actual manufacturing location.
In the calibration apparatus, moments to be measured by the multi-component force gauge are calibrated as described below. That is, a long-shaped force transmitting member is extended from an end of a spindle shaft along an axial direction thereof, and a rope is installed on an end region of the force transmitting member. A weight is tied to an end of the rope via a tackle, to thereby exert an upward force Fz on the end region of the force transmitting member. In this way, a moment Mx corresponding to a product of a load Fz of the weight and a length L of the force transmitting member is exerted together with the upward force Fz about an X axis on a tire mounting location. For causing the multi-component force gauge to accurately detect only the moment, measurement must be performed under a condition that the upward force Fz is absent. Thus, in order to cancel the force Fz, a weight, which is as heavy as the above-described weight, is tied via a rope to the tire mounting location, thereby exerting a downward force Fz.
By doing so, only the moment Mx is exerted on the multi-component force gauge, which allows the moment Mx to be calibrated in the multi-component force gauge.
In this connection, the conventional moment calibration method requires, as described above, to exert loads, which are parallel in vertically opposite directions to each other, on the force transmitting member in either of the calibration apparatus for separately calibrating the multi-component force gauge or the calibration apparatus for calibrating the multi-component force gauge that is in the state incorporated in the tire spindle. To achieve this, it is necessary for both of the calibration apparatus to include, as illustrated in FIG. 4, the pulley for pulling the rope along a predetermined (upward in FIG. 4) direction.
The use of the pulley, however, causes a frictional force to arise between the pulley and the rope and accordingly reduces accuracy in moment calibration due to the frictional force. In addition, when the pulley is used, friction on a sliding surface (a bearing) of the pulley also contributes to the reduction in accuracy. Further, in the example shown in FIG. 4, because the moment Mx is not accurately exerted on the multi-component force gauge when the load applied to the force transmitting member is deviated from the vertical direction, a location to install the pulley should be exactly adjusted above the force transmitting member along the vertical direction. This location adjustment of the pulley, which is a delicate and time-consuming task, has impaired workability in moment calibration.
Moreover, because the rope or wire is typically twisted, a torsional force caused by the twisting is also exerted on the force transmitting member. As the loads become greater, the torsional force is increased and may be added as an error to a measurement in the multi-component force gauge. Accordingly, in the moment calibration method using the rope or wire, great hysteresis results from the frictional force or the torsional force described above, and it has been impossible due to the hysteresis to perform moment calibration with a high degree of accuracy.